The present invention relates to a time-division multiplexing data transmitting and receiving system which is applicable to a vehicle and which has a high data transmission reliability.
A U.S. Pat. No. 4,370,561 issued on Jan. 25, 1983 exemplifies a conventional time-division multiplexing system.
In the above-identified U.S. Patent document, a vehicle power supply (car battery) is connected to each transmit and receive unit, a synchronizer (multiplex timing unit), and loads via a circuit protector and a power supply line.
The synchronizer outputs a synchronization signal via a communication line to each receive and transmit unit in the vehicle. The synchronization signal is a pulse train having a pulse period defining a master synchronization interval. The transmit units and the receive units count negative-going pulses subsequent to the pulse defining the master synchronization interval to detect a transmission/reception channel. Subsequent to the negative-going pulse, the synchronization signal provides another pulse, the period thereof defining a data transmission interval. During the data transmission interval, the output state of the synchronizer is in a floating state (high impedance). One of the transmit units outputs data in the form of a high ("H") level or a low ("L") level during the data transmission interval according to an input state from a switch, which corresponds to a predetermined channel when the data transmission/reception channel, specified sequentially by the negative-going pulse, indicates the predetermined channel.
On the other hand, one of the receive units detects the predetermined channel in the same way as the transmit unit described above. A voltage level on the communication line during the transmission interval is latched in response to a timing pulse. A relay connected to the receive unit is turned ON in response to the latched output so that the load is actuated. It is noted that after the end of the transmission interval, the synchronizer outputs the synchronization signal having an interval indicating "H" level.
In the system described above, when a plurality of communication channels are generated on the communication line at the same time, the data transmission from the transmit unit to the receive unit can be carried out on the same line.
However, in the conventional time-division multiplexing system, the output state of the synchronizer is a tri-state level. Therefore, logic circuitry for detecting the floating interval derived from the synchronizer becomes necessary for each transmit unit and receive unit. Consequently, the whole circuit construction of each transmit unit and receiver unit becomes complex. In addition, each synchronizer, transmit unit, and receive unit requires an expensive accurate clock generator. Therefore, the manufacturing cost of the whole multiplexing system becomes high.
Furthermore, the voltage level becomes unstable during the transmission interval if breakage of the communication line occurs in the vicinity of any one of the transmit units. Consequently, one of the transmit units will fail to read correct data.
On the other hand, since the output of the transmit units is a CMOS (complementary MOS) output, a single switch can only access the same transmission channel and data collision is not permitted since an intermediate level between the "H" and "L" levels may result, depending on the switched states of two or more switches. Hence, it is necessary to provide a wired-OR on a conventional wire harness in the vehicle. For example, in a case where two or more control switches for the same piece of equipment (radio and so on) are provided at two or more positions of the vehicle, double channels need to be prepared for transmitting the switch signals indicating the same meaning. The receive unit needs to receive the channels as wholly different double signals and therefore needs to provide a logical OR. Consequently, data transmission efficiency is hampered.
Furthermore, when the communication line is short-circuited (slight term short) in the "H" level output state such as occurs in the master synchronization interval, the short-circuited state is detected in the same way as the detection of a negative-going pulse used for the channel designation. Therefore, the channel counting in the transmit and receive units becomes erroneous. Consequently, a failure in transmission of data between the transmit and receive units may easily occur. In this way, reliability of data communication is reduced in the case when the above-described system is applied to the actual wire harness of the vehicle.